Alloying at a Subnanoscale Maximizes the Synergistic Effect on the Electrocatalytic Hydrogen Evolution

Abstract Bonding dissimilar elements to provide synergistic effects is an effective way to improve the performance of metal catalysts. However, as the properties become more dissimilar, achieving synergistic effects effectively becomes more difficult due to phase separation. Here we describe a comprehensive study on how subnanoscale alloying is always effective for inter‐elemental synergy. Thirty‐six combinations of both bimetallic subnanoparticles (SNPs) and nanoparticles (NPs) were studied systematically using atomic‐resolution imaging and catalyst benchmarking based on the hydrogen evolution reaction (HER). Results revealed that SNPs always produce greater synergistic effects than NPs, the greatest synergistic effect was found for the combination of Pt and Zr. The atomic‐scale miscibility and the associated modulation of electronic states at the subnanoscale were much different from those at the nanoscale, which was observed by annular‐dark‐field scanning transmission electron microscopy (ADF‐STEM) and X‐ray photoelectron spectroscopy (XPS), respectively.


Table of Contents
Section S1. Experimental section S1.1 Chemicals S1.2 Preparation of SNPs and NPs S1.3 Characterization of SNPs and NPs S1.4 Electrochemical measurements for the hydrogen evolution reaction (HER) S1.5 Processing of ADF-STEM images Section S2. APD conditions for preparation of SNPs and NPs Table S1. APD deposition conditions Section S3. STEM images and elemental mapping of bimetallic SNPs and NPs                                            Table S4. Most stable cluster structures and the lowest H adsorption energies (Eads) against composition ratio. Green, gray, and pink spheres indicate Zr, Pt, and H atoms, respectively. The threshold value for bond length between metal atoms is 3 Å. Figure S47. Effect of the number of Pt atoms in the SNP on Eads. In each case, total number of atoms in one SNP was fixed to six, and the number of Zr atoms is given as the difference between six and the number of Pt atoms. Section S1. Experimental Procedures S1.1 Chemicals.
The pretreatment of graphene nanoplatelets was conducted as reported previously [1] . The graphene nanoplatelets purchased from Aldrich were washed with aq. HCl (300 mL, 3 mol mL -1 ), CH3OH, and purified water in sequence to remove the impurities. The CH3OH in this study was purchased from Kanto Chemical Co., Inc. and the H2SO4 was purchased from Fujifilm Wako Pure Chemical Co., and both were used as received.

S1.2 Preparation of SNPs and NPs
The SNPs or NPs were prepared on graphene, a silicon plate, or a glassy carbon electrode (GCE) using a vacuum deposition method with a pulsed arc plasma source (Advance Riko, APS-1) equipped with metal cylinder targets. The arc pulse was generated with a frequency of 1 Hz with a period of 500 ms and current amplitude of 2 kA. The deposition amount of each metal element was determined by the number of pulses based on the amount of deposition calculated by the quartz crystal microbalance (QCM) method. Discharge voltage and capacity were adjusted according to the respective targets, as shown in Table S1.

S1.3. Characterization of SNPs and NPs
Annular dark-field scanning transmission electron microscopy (ADF-STEM) images were obtained using an aberration-corrected transmission electron microscope (Jeol, JEM-ARM200F) operated at an 80 kV acceleration voltage. The inner and outer collection angles used in recording the ADF-STEM images were 57 and 226 mrad, respectively. Probe current was fixed at 26 pA. The SNPs and NPs were processed directly onto graphene nanoplatelets using arc-plasma deposition (APD). The graphene nanoplatelets were deposited on thin holey carbon-film-coated Cu grids (Nissin EM Co., Ltd.) by drop-casting of the homogeneous suspension in methanol, followed by vacuum drying overnight.
X-ray photoelectron spectroscopy (XPS) was conducted with a Shimadzu ESCA-3400HSE instrument with Mg Kα X-ray (10 kV, 20 mA). The silicon substrates were cleaned in H2SO4 with ultrasonication for 15 minutes, rinsed with ultrapure water three times, and vacuum dried overnight. Various SNPs and NPs for XPS analysis were deposited on silicon substrates under the APD conditions shown in Table S1. S1.4. Electrochemical measurements for the hydrogen evolution reaction (HER) Electrochemical measurements were obtained using an electrochemical analyzer (ALS750A, CH Instruments) in standard threeelectrode configuration. A platinum wire and reversible hydrogen electrode (RHE) were used as a counter electrode and reference electrode, respectively. The working electrode was a catalyst-modified glassy carbon disk electrode (GCE, 3.0 mm diameter). The SNPs and NPs were deposited directly on the GCE. Before deposition, the GCE was polished with 1-, 0.3-, and 0.005-µm alumina paste. The APD-processed electrode was used only once for electrochemical measurements. A typical HER measurement was conducted in 0.05 M H2SO4 electrolyte after argon bubbling for 15 min. Electrochemical cleaning of the catalyst surface deposited on the working electrode was conducted by scanning the working electrode potential from 0.06 to 1.00 V vs. RHE with a scan rate of 0.1 V s -1 for 21 segments. Next, cyclic voltammetry (CV) was conducted from 0.06 to 1.00 V at a scan rate of 50 mV s -1 for five segments. Finally, a Tafel plot was obtained from −0.3 to 0.2 V vs. RHE at a scan rate of 2 mV s -1 . The polarization curves were expressed as overpotential (h) vs. log current [log(i)] to create the Tafel plots [2] . By fitting the linear portion of the Tafel plots to the Tafel equation [h = A log(i/i0)], the Tafel slope (A) was obtained. The exchange current is i0, which represents the intrinsic HER activity of catalysts under reversible conditions. The value of i0 for the catalyst was obtained by extrapolating the zero tangent line to 0.0 V vs. RHE, which is the zero overpotential of HER. The resistance of our experimental setup (0.05 M H2SO4 solution) was 67 Ω. Therefore, the IR drop in the current range observed in our experiments (<100µA) was less than 7mV, which was within the experimental error. (Fig. S43) S1.5. Processing of ADF-STEM images All successive 50 frames in the ADF-STEM movie (Movie 1) were processed by following four steps using Image J software [3,4] . First, the original images acquired from STEM were upconverted from to 512 × 512 pixels to 1024 × 1024 pixels using the resize method (interpolation=Bicubic). Second, the background of resized images was subtracted using the Subtract Background method (radius=50). Third, the Gaussian Blur method (sigma=4) was applied to remove high-frequency noise from the images. Fourth, the xy coordinates and the intensity values for local brightness maxima were searched by applying the Find Maxima method (prominence=0) to extract the positions and elements of atoms. The brightness intensity value was used to identify the element (Pt or Zr) in the images. The spots with an intensity lower than 3800 were removed from the list because they were considered background noise. The intensity histogram showed two maxima at 5050 and 9210 (Fig. S46), which correspond to Zr and Pt atoms, respectively. Therefore, the intermediate intensity was set as the threshold for elemental identification, and those with an intensity higher than the threshold were assigned to Pt, while those with a lower intensity were assigned as Zr. Extraction of chemical bonds from the list of atomic coordination values were conducted from all atomic pairs in the same frame. The threshold distance of bonds dth was set to 0.35 nm. The movie visualizing atom mapping (colored circles) and chemical bonds (white line) (Movie 2) was made based on the extracted coordinates (x, y) of element and chemical bonds. The overlayed movie (Movie 3) of Movies 1 and 2 represents validation of the analysis.
Here, the numbers of atoms ( , ) and bonds ( % , % , % ) for each frame were counted and the molar fraction of Pt in a field of view was calculated according to eq. S1. The ratio of each bond to each frame was calculated using the number of bonds (eqs. S2-S4). ) (S1) Section S2. APD conditions for preparation of SNPs and NPs . .      . Figure S6. ADF-STEM and particle size histograms of Pt3W3 SNPs and Pt10W10 NPs.
. Figure S11. ADF-STEM and particle size histograms of Pd3Mo3 SNPs and Pd10Mo10 NPs Figure S12. ADF-STEM and particle size histograms of Pd3W3 SNPs and Pd10W10 NPs.   . Figure S17. ADF-STEM and particle size histograms of Ru3W3 SNPs and Ru10W10 NPs.                     Three relationships are possible between compositional ratio and HER activity. As shown in Figure S45, they were categorized as positive synergistic effect, negative synergistic effect, or no synergistic effect. [5], [6] To quantify and summarize all possible relationships, the HSI was used.

Section S8. DFT calculations
To explore the most stable structure metastable structures of PtxZry SNPs (x + y = 6), the CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) program [7][8][9] and Gaussian 16 [10] program package were used. CALYPSO is a program based on the particle swarm optimization (PSO) algorithm and is used for global optimization. Gaussian 16 is used for local optimization. The structure of each SNP is roughly optimized using the PM7 [11] method and then optimized accurately by B3LYP [12][13][14] functional and lanl2dz [15][16][17] basis sets. The spin multiplicity considered was in range from singlet to dectet. Then the adsorption structure of a hydrogen atom was optimized on the SNP with the lowest formation energy in each composition ratio. B3LYP/lanl2dz was used for the optimization. All adsorption structures were considered and the most stable adsorption site was explored.